Japanese Laid-Open Patent Publication No. 2003-20930 describes a technique for burning and eliminating particulate matter (PM) accumulated in a filter, which is arranged in the exhaust system of a diesel engine, when the amount of PM accumulated in the filter exceeds a predetermined amount. The PM accumulated in the filter is burned and eliminated by heating the filter and intermittently adjusting the air-fuel ratio to the lean side. To be specific, the air-fuel ratio is intermittently adjusted to the lean side by intermittently adding fuel to the exhaust (intermittent fuel addition). In some cases, the air-fuel ratio may be intermittently adjusted to the lean side by intermittently increasing the amount of fuel supplied to the exhaust (intermittent increase of fuel addition).
In this prior art arrangement for eliminating PM, the amount of active oxygen is adjusted by changing the fuel supply interval based on the PM accumulation amount. In this way, the PM is eliminated through oxidation while fuel efficiency is prevented from decreasing. In this prior art arrangement, however, the temperature of the filter is not taken into consideration when the fuel supply interval is changed. Thus, depending on the clogging degree of the filter or the driving state of the diesel engine, the filter may be overheated. This would cause thermal deterioration of the filter.
To avoid this, the interval for supplying fuel needs to be adjusted based on the temperature of the exhaust that passed through the filter. However, the temperature of the filter does not increase immediately after the fuel supply to the exhaust system is started. Further, the temperature of the filter does not decrease immediately after the fuel supply to the exhaust system is stopped. That is, a time delay exists between when the fuel supply is started or stopped and when the temperature of the filter changes. Such a temperature characteristic differs depending on the deterioration degree of the catalyst in the filter.
Thus, if the fuel supply to the exhaust system is simply controlled based on the temperature of the exhaust that passed through the filter, the filter may be overheated. For example, although the fuel supply is stopped assuming that there is still time before the filter is overheated, the temperature of the filter further increase and the filter may be overheated.
When the sensor detecting the temperature of the exhaust deteriorates, or when the exhaust discharged from the filter flows unevenly, the detection response of the temperature of the exhaust that has passed through the filter may be delayed or the detection accuracy of the temperature of the exhaust that passed through the filter may be degraded. In such a case, the fuel supply is controlled based on a detection value differing from the actual temperature of the exhaust. As a result, the fuel supply is started or stopped at inappropriate timings. This may cause the filter to be overheated.
In particular, when burn-up heating for completely burning PM is performed by intermittent fuel addition to the exhaust system or intermittent increase of fuel addition to the exhaust system, overheating of the exhaust purification apparatus caused by the time delay of the temperature increase or the detection error of the temperature is remarkable.